This application claims the priority of German Patent Application Serial No. 100 59 195.7, filed Nov. 29, 2000, the subject matter of which is incorporated herein by reference.
The present invention relates to an arrangement for treatment of exhausts released from an Otto engine with direct fuel injection.
Modern Otto engines with direct fuel injection are characterized by lean-combustion operation or understoichiometric operating points in order to realize a lowest possible fuel consumption. As a consequence, current catalytic converters, in particular three-way catalytic converters, are not capable to completely convert the harmful components hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) into non-toxic compounds such as carbon dioxide (CO2), nitrogen (N2) or water (H2O). The reason for the incomplete conversion of the harmful components is low exhaust temperatures and higher HC-emissions-during lean-combustion operation of the Otto engines, in particular the need for NOx-reduction in the oxygen-rich exhaust gas.
For environmental reasons, a particular exhaust treatment is desired in order to achieve NOx efficiencies of above 90% at a temperature range between 300xc2x0 C. and 500xc2x0 C. Proposals have been made to integrate three-way catalytic converters in the exhaust line near the engine and NOx-adsorbers underneath the bottom of the motor vehicles. Such arrangements must, however, reconcile two contradicting requirements. On the one hand, the three-way catalytic converters should be raised in shortest possible time to an operating temperature of  greater than 350xc2x0 C. On the other hand, the exhaust temperature should not exceed 500xc2x0 C. to prevent adverse effect on the maximum temperature resistance of the NOx-adsorbers.
As far as a short heating time for the catalytic converters is concerned, this can be implemented by using so-called air-gap insulated exhaust guiding elements, such as exhaust manifolds or front pipes. These elements have a double-walled configuration and include thin-walled interior parts (exhaust pipes) with slight heat storage capacity, resulting in a low heat withdrawal from the exhaust and thus to a rapid heating of the catalytic converters, and exterior parts (outer tubes), which engender the tightness and provide also a load-carrying function.
To date however, the second requirement has been difficult to meet in a satisfactory manner because of the presence of temperatures of well above 500xc2x0 C., in particular in the high-load range or high-speed range. As a consequence, it has been proposed to provide fairly long exhaust lines, thereby cooling the exhausts on their way to the NOx-adsorbers. Still, the temperature stress remains very high so that the service life of currently available NOx-adsorbers is insufficient.
It is also known to provide a cooling during supply of fuel. Such an engine-internal measure significantly deteriorates, however, the overall efficiency.
German Pat. No. DE 693 09 453 T2 describes a thermally insulating jacket under reversible vacuum for use, among others, with catalytic silencers of motor vehicles and trucks. A metal powder may be provided in the jacket for release of hydrogen.
German Pat. No. DE 196 04 367 A1 describes an exhaust pipe, which has an inner tube which can be moved inside an outer tube and defines with the outer tube an annular air gap. The outer tube is associated with an extension compensator which is formed by an hydraulic bellows-type deformation of the wall of the outer tube.
It would be desirable and advantageous to provide an arrangement for treatment of exhausts released from an Otto engine with direct fuel injection, which arrangement is able to reconcile contradictory requirements with respect to the use of a controlled catalytic converter with downstream NOx adsorber in an exhaust line.
According to one aspect of the present invention, an arrangement for treating exhaust emitted from an Otto engine with direct fuel injection, includes an engine-proximate catalytic converter; a NOx adsorber; an exhaust pipe connecting the NOx adsorber with the catalytic converter; an outer tube having a flexible length portion in the form of a metallic bellows and surrounding the exhaust pipe at formation of a circumferential gap therebetween; and a metal powder received in the gap and releasing or adsorbing hydrogen gas in dependence on a temperature in the gap, wherein the gap is evacuated to provide a heat-insulating atmosphere.
In accordance with the present invention, a heat-insulating environment is initially produced in the gap between the exhaust pipe and the outer tube, by evacuating the gap or drawing a vacuum. Through the provision of such a vacuum, the resultant insulating action causes a fastest possible heating of the catalytic converter as a consequence of a lack of heat transport through convection so that also only small amounts of harmful emissions are generated. This atmosphere has to be created only once and is maintained over the entire service life of the arrangement.
In concert with the creation of the heat-insulating atmosphere, the gap may also contain a particular metal powder which releases hydrogen gas at a temperature above approximately 450xc2x0 C. to 500xc2x0 C. As a result, the insulating effect of the atmosphere is negated so that the hydrogen gas, which is highly effective for transport of heat, can transfer heat from the exhaust pipe to the outer tube and ultimately to the ambient air. Thus, the NOx adsorber will not be exposed to temperatures of above about 500xc2x0 C. and its temperature resistance remains uninhibited.
The present invention resolves prior art problems by attaining a rapid heating of the catalytic converter and a heat transport at elevated temperatures. The use of a metal powder by which hydrogen gas is released or re-adsorbed permits a control and adjustment of the atmosphere in the gap at particular temperatures. The composition of the metal powder is selected in dependence on the required temperature to be controlled. Examples of suitable metal powders include an alloy which contains, i.a., zirconium, vanadium and iron. The amount being used is dependent on the situation in the Otto engine at hand, on the size of the installation space available in the motor vehicle, and on the configuration of the exhaust line.
As the outer tube has normally a different temperature than the temperature of the inner exhaust pipe, a length portion of the outer tube is axially flexible to minimize occurring mechanical tension. Suitably this length portion is provided in proximity of the catalytic converter and configured as metallic bellows.
An arrangement according to the present invention allows production of an efficient drive system with reduced fuel consumption at lower emissions. In particular the service life of the NOx adsorber is greatly extended. The functional separation between the catalytic converter and the NOx adsorber allows the use of particular materials so that longer service life can be established for the exhaust pipe and the outer tube between catalytic converter and NOx adsorber, while still being able to reduce the weight.